1. Field of the Invention
The present invention relates to a lens evaluation device, and more particularly to a lens evaluation device for evaluating the performance related to an image position of an optical system, specifically lateral magnification, distortion aberration, field curvature and chromatic aberration.
2. Description of the Related Art
When measuring an optically picked-up image, highly accurate optical aberration correction is necessary. For example, in the case of a laser confocal microscope, a depth direction cannot be accurately measured since the observation image of a flat sample curves if there is field curvature. Or when picking up images by laser with a plurality of wavelengths and comparing them, an image position slides in a lateral direction (direction perpendicular to the optical axis of the optical system) and a longitudinal direction (the optical axis direction of the optical system) depending on a wavelength if there is chromatic aberration in the optical system. Therefore, accurate comparison/operation is impossible. Similarly, the error of the lateral magnification and distortion aberration cause errors in the measurement of the image position. In order to correct these errors, such aberration is measured in advance and a picked-up image must be modified by the measured aberration. Alternatively, the lens position of the optical system is modified by the measured aberration and the aberration must be reduced to sufficiently small. In such a case, an evaluation device capable of measuring the performance related to an image position of the optical system, specifically lateral magnification, distortion aberration, field curvature and chromatic aberration with high accuracy is indispensable.
The conventional evaluation device is described below with reference to FIG. 1.
As shown in FIG. 1, in the conventional evaluation device, a pin-hole 51, which becomes a point light source, is installed on the object surface of an optical system 52 to be evaluated and is illuminated from behind by an illumination means, which is not shown in FIG. 1. The aerial image 53 of the pin-hole 51 is formed on the image surface of the optical system 52. However, since this aerial image 53 is small, an image position with the maximum intensity cannot be measured with sufficient accuracy even if the aerial image is directly picked up by an imaging device. Therefore, its image enlarged by an enlargement optical system 54 is picked up by the imaging device 55. Thus, its image position can be determined by retrieving a pixel with the maximum brightness from the enlarge image. Furthermore, if the image is picked up while moving the enlargement optical system 54 and the imaging device 55 in the optical axis direction (Z axis direction) of the optical system 52 and a pixel with the maximum brightness is retrieved from its stack image, the X, Y and Z coordinates of the image position can be determined. By switching the wavelength of the illumination means, chromatic aberration can also be evaluated. However, in order to calculate the X, Y and Z coordinates of the image position, the respective positions of the enlargement optical system 54 and the imaging device 55 must be monitored by three pieces of length meter. In order to determine the shape of the entire object within the view field, the image must be measured while moving the pin-hole 51 to a plurality of positions on the object surface. In this case, the X, Y and Z coordinates of the pin-hole 51 must also be monitored by the length meter.
As the prior art related to the lens evaluation device in this technical field, there are Patent references 1-5 and Non-patent reference 1.
Patent reference 1: Japanese Patent No. 3391470
Patent reference 2: Japanese Patent Application No. 2002-289494
Patent reference 3: Japanese Patent Application No. 2004-163207
Patent reference 4: Japanese Patent Application Publication No. H6-21772
Patent reference 5: Japanese Patent Application No. H1-270605
Non-patent reference 1: “Image Correction for Highly Accurate Image Measurement Using Digital Image”, by Nakamura et al., Journal of the Electronic Image Association, Vol. 31, No. 4, pages 534-541 (Jul. 25, 2002).